1. Technical Field
The present invention relates to a wavelength variable interference filter, an optical filter device, an optical module, and an electronic apparatus.
2. Related Art
In the past, there is known a wavelength variable interference filter in which reflective films are respectively arranged to be opposed to each other via a predetermined gap on surfaces opposed to each other of a pair of substrates (see, for example, JP-A-2002-277758 (Patent Literature 1)).
In the wavelength variable interference filter disclosed in Patent Literature 1, reflective layers (reflective films) are respectively provided on surfaces opposed to each other of two optical substrates. Plural actuators are arranged between the optical substrates to make it possible to change the distance between the optical substrates. Further, in the wavelength variable interference filter, capacitive electrodes are respectively provided on the surfaces opposed to each other of the two optical substrates to make it possible to grasp a space between the substrates on the basis of charges stored in the capacitive electrodes.
The wavelength variable interference filter disclosed in Patent Literature 1 can measure the space between the substrates by measuring the charges stored in the capacitive electrodes. In this case, it is necessary to apply a feeble voltage between the capacitive electrodes and cause the capacitive electrodes to store the charges. However, since the charges stored in the capacitive electrodes escape to the surfaces of the substrates and the reflective films, the surfaces opposed to each other of the substrates are sometimes charged.
In general, three reasons explained below are conceivable as a reason why the charges of the capacitive electrodes escape to the surfaces of the substrates and the reflective films. First, when the surfaces of the substrates are made of a material that is easily charged, electrostatic induction is caused by an electric field generated from ends of the capacitive electrodes. The surfaces of the substrates are charged by the electrostatic induction. Second, when insulation properties of the surfaces of the substrates are low, a leak current flows on the surfaces of the substrates from the capacitive electrodes to charge the surfaces of the substrates. Further, it is conceivable that the leak current flows to the reflective films to charge the reflective films. Third, when the reflective films and the capacitive electrodes are close to each other, since capacitive coupling occurs between the capacitive electrodes and the reflective films, the reflective films are charged through the capacitive coupling.
When the substrates opposed to each other are charged in this way, a Coulomb force acts between the substrates. Therefore, it is difficult to control the distance between the reflective films.